Can giant radio halos probe the merging rate of galaxy clusters?
Abstract
Radio and X-ray observations of galaxy clusters probe a direct link between cluster mergers and giant radio halos (RH), suggesting that these sources can be used as probes of the cluster merging rate with cosmic time. In this paper we carry out an explorative study that combines the observed fractions of merging clusters (fm) and RH (fRH) with the merging rate predicted by cosmological simulations and attempt to infer constraints on merger properties of clusters that appear disturbed in X-rays and of clusters with RH. We use morphological parameters to identify merging systems and analyze the currently largest sample of clusters with radio and X-ray data (M500>6d14 Msun, and 0.2<z<0.33, from the Planck SZ cluster catalogue). We found that in this sample fm~62-67% while fRH~44-51%. The comparison of the theoretical fm with the observed one allows to constrain the combination (xim,taum), where xim and taum are the minimum merger mass ratio and the timescale of merger-induced disturbance. Assuming taum~ 2-3 Gyr, as constrained by simulations, we find that the observed fm matches the theoretical one for xim~0.1-0.18. This is consistent with optical and near-IR observations of clusters in the sample (xim~0.14-0.16). The fact that RH are found only in a fraction of merging clusters may suggest that merger events generating RH are characterized by larger mass ratio; this seems supported by optical/near-IR observations of RH clusters in the sample (ximin~0.2-0.25). Alternatively, RH may be generated in all mergers but their lifetime is shorter than τm (by ~ fRH/fm). This is an explorative study, however it suggests that follow up studies using the forthcoming radio surveys and adequate numerical simulations have the potential to derive quantitative constraints on the link between cluster merging rate and RH at different cosmic epochs and for different cluster masses.